(1) Field of the Invention
The present invention relates to a method and a system for generating accessory power from a gas turbine engine. In particular, the present invention relates to a hybrid engine accessory power system that enables improved gas turbine engine operability characteristics.
(2) Prior Art
Horsepower extraction from a gas turbine engine typically incorporates a mechanical gearbox that is driven by a power takeoff shaft that is directly connected to one of the main drive shafts in the engine. The gearbox is mounted in such a way as to facilitate the subsequent attachment of all of the engine-driven accessories such as a fuel pump, an oil pump, a hydraulic pump, electrical generators, etc. The gearbox represents the transfer of gas turbine mechanical shaft power to accessory mechanical shaft power.
Gas turbine engine high pressure compressors operate steady state along an operating line 10 of increasing flow and pressure ratio at increasing rotor speed as shown in FIG. 1. A compressor's limiting operability characteristic is the stall line 12 beyond which stable compressor airflow cannot be sustained. A compressor operating line 10 at a given airflow is lower in pressure ratio than the stall line 12 to provide a margin for engine transient operation. During engine acceleration, the compressor deviates from the steady state operating line 10 and moves along a transient operating line 14. For the typical high-pressure compressor, the transient operating line 14 during acceleration is characterized by reduced stall margin across the engine operating range. Accessory power demand negatively affects transient operation by reducing the amount of stall margin available as illustrated in FIG. 2.
Gas turbine accessory power has been provided by mechanical means through a series of gear sets and transmission shafts attached to the engine's high pressure rotor. Electrical and hydraulic power for airplane systems, along with motive power for the engine oil and fuel pumps, are provided by the engine mounted accessory power train. High levels of shaft power extraction decreases the amount of stall margin available for engine transient operation as depicted in FIG. 2.
There are several options available allowing transient operation within these limitations. The rate of engine acceleration can be slowed; however, this may be incompatible with the demands of aircraft safety during emergency circumstances such as obstacle avoidance. Raising the minimum idle high rotor speed, increasing idle thrust, thereby reducing the idle to max power thrust range, also allows a lower acceleration rate and transient excursion. Again, this may be incompatible with aircraft operation as higher idle speed results in a higher idle thrust, which requires higher airplane drag to descend. Given airplane idle requirements for descent profile and engine thrust response for airplane safety, the compressor transient excursion is essentially fixed, requiring some relief with regard to accessory power effects on the high pressure rotor.
Compressor bleed air can be used to drop the operating line of the compressor away from the surge line. This technique is commonly used; however, it has several drawbacks such as increased engine noise impact, and compatibility of the high temperature exhaust with composite engine cowl structures.
There is a need for a system which simultaneously allows for reduced mechanical shaft power load and systems capacity to absorb and utilize the energy of compressor bleed air at low power.